Rhodamine 19 Fluorescent Dimers Resulting from Dye Aggregation

Rhodamine 19 (R19) doped silica glasses were prepared through the sol-gel ... plays on the R19 aggregation process was studied through excitation and ...
0 downloads 0 Views 108KB Size
Download PDF
2782

Langmuir 2003, 19, 2782-2786

Rhodamine 19 Fluorescent Dimers Resulting from Dye Aggregation on the Porous Surface of Sol-Gel Silica Glasses Maria L. Ferrer, Francisco del Monte,* and David Levy† Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Cientı´ficas (ICMM-CSIC), Cantoblanco, Madrid, 28049, Spain Received October 11, 2002. In Final Form: January 24, 2003

Rhodamine 19 (R19) doped silica glasses were prepared through the sol-gel process with dye concentrations ranging from 4.0 × 10-6 to 6.4 × 10-4 M. The role that the porous surface of the silica glasses plays on the R19 aggregation process was studied through excitation and emission fluorescence spectroscopy, absorption spectroscopy, and fluorescent lifetime measurements. The photophysical behavior of R19 at intermediate concentration values revealed the formation of fluorescent J-dimers in an in-plane oblique angle configuration. The exciton theory was used for the elucidation of the adsorption angle (φ), the angle between the monomer units (R), and the separation distance between the molecules (R) of the R19 fluorescent dimers. Further increase of R19 concentration was observed to promote the conversion from fluorescent to nonfluorescent dimers (H-dimers) through a rearrangement of the monomeric constituents to obtain a parallel plane twist angle configuration.

Introduction The sol-gel process is a chemical route for the preparation of porous inorganic matrixes (e.g., silica) under mild conditions.1 The ability of the resulting matrixes to provide a friendly environment to a variety of organic molecules,2 macromolecules,3 and even biomolecules4 makes these materials of remarkable interest for the preparation of number of hybrid organic-inorganic materials.5 Knowledge regarding the physical and chemical characteristics of the environment surrounding the molecules is crucial for the understanding of the molecular organization in the resulting hybrid materials. However, limited information is available regarding the chemical interactions that the molecules establish among themselves and with the porous cage of the matrix where they are trapped.6-8 This mainly results from a paucity of tools capable of probing the surface and its interactions specifically. Of particular interest are the aggregation processes that occur on a surface upon the increase of the molecule concentration. The aggregation of rhodamine molecules absorbed on both organic and inorganic supports has been widely reported.9-12 In the absorbed state, the electrostatic interaction of the molecule with the polar surface and * Corresponding author. Fax: +34 91 372 0623. E-mail: [email protected]. † Also at Instituto Nacional de Tecnologı´a Aeroespacial-INTA, Laboratorio de Instrumentacio´n Espacial-LINES, Torrejo´n de Ardoz, Madrid, 28850, Spain. (1) Brinker, C. J.; Scherer, G. W. Sol Gel Science; Academic Press: San Diego, 1990. (2) Avnir, D.; Levy, D.; Reisfeld, R. J. Phys. Chem. 1984, 88, 5956. (3) Pope, E. J. A.; Mackenzie, J. D. J. Non-Cryst. Solids 1986, 87, 185. (4) Avnir, D.; Braun, S.; Lev, O.; Ottolenghi, M. Chem. Mater. 1994, 6, 1605. (5) Sanchez, C.; Ribot, F. New J. Chem. 1994, 18, 1007. (6) Leveau, B.; Herlet, N.; Livage, J.; Sanchez, C. Chem. Phys. Lett. 1993, 206, 15. (7) Narang, U.; Wang, R.; Prasad, P. N.; Bright, F. V. J. Phys. Chem. 1994, 98, 17. (8) Marchi, M. C.; Bilmes, A. A.; Negri, R. M. Langmuir 1997, 13, 3665.

with other monomer molecules governs the formation of aggregates.13-16 The interfacial ordering adopted by the monomer constituents on the surface of adsorption results in the formation of aggregates with different configurations; for example, parallel plane twist angle and in-plane oblique angle.17-19 The fluorescent or nonfluorescent (Jdimers and H-dimers, respectively) character of the resulting dimers is determined by such a geometry. For the in-plane oblique angle configuration, the geometry is basically defined by the angle between the monomer units (R), the separation distance between the molecules (R), and the angle formed between the longitudinal axis of the molecules (monomer transition moments) and the adsorption surface (φ).13,17 Thus, values of φ ranging from 0 to 54.7° are required for fluorescent J-dimer formation while over those angle values (φ > 54.7°), nonfluorescent H-dimers are formed.13-15 Any of these configurations are characterized by unequivocal spectroscopic properties. The aim of this work is to study the role played by the porous surface of sol-gel silica glasses on the formation of different rhodamine 19 (R19) dimers. The fluorescent or nonfluorescent character of the dimers and the preferred configuration adopted by any of the different dimers will be studied for a wide range of R19 concentrations. The (9) Vieria-Ferreira, L. F.; Cabral, P. V.; Almeida, P.; Oliveira, A. S.; Bothelo do Rego, A. M. Macromolecules 1998, 31, 3936 and references therein. (10) Slyadneva, O. N.; Slyadnev, M. N.; Tsukanova, V. M.; Inoue, T.; Harata, A.; Ogawa, T. Langmuir 1999, 15, 8651 and references therein. (11) Chaudhuri, R.; Lopez-Arbeloa, F.; Lopez-Arbeloa, I. Langmuir 2000, 16, 1285 and references therein. (12) Nishikiori, H.; Fujii, T. J. Phys. Chem. B 1997, 101, 3680 and references therein. (13) Kemnitz, K.; Tamai, N.; Yamazaki, I.; Nakashima, N.; Yoshihara, K. J. Phys. Chem. 1986, 90, 5094. (14) Kemnitz, K.; Yoshihara, K. J. Phys. Chem. 1991, 95, 6095. (15) Itho, K.; Chiyokawa, Y.; Nakao, M.; Honda, K. J. Am. Chem. Soc. 1984, 106, 1620. (16) Muto, J. J. Phys. Chem. 1976, 80, 1342. (17) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. Pure Appl. Chem., 1965, 11, 371. (18) McRae, E. G.; Kasha, M. Physical Processes in Radiation Biology, 1st ed.; Academic Press: New York, 1964. (19) McRae, E. G.; Kasha, M. J. Chem. Phys. 1961, 11, 38.

10.1021/la026685t CCC: $25.00 © 2003 American Chemical Society Published on Web 02/21/2003

Fluorescent Dimers from Dye Aggregation

Langmuir, Vol. 19, No. 7, 2003 2783

Table 1. Moles of R19 Incorporated during the Sol-Gel Process and R19 Concentration of the Resulting Silica Glasses (Gels 1-8) gel 1 moles of R19 × [R19] (M × 106)

107

2

3

4

5

6

7

8

0.03 0.075 0.3 0.6 1.2 2.4 3.6 4.8 4 10 40 80 160 320 480 640

structural parameters of the in-plane oblique angle configuration (e.g., R, φ, and R) will be determined through the application of the exciton theory17-19 to provide a complete view of the geometry adopted by the R19 fluorescent J-dimers on the porous surface of sol-gel silica glasses. Experimental Section Materials. Rhodamine 19 (R19, from Lambda Physics) was used as received without further purification. Tetramethyl orthosilicate (TMOS, from Aldrich), spectrophotometric grade methanol (from Merck), and distilled and deionized water (DDW) were also used in the sample preparation. Sample Preparation. Silica gels (gels 1-7) were prepared from 9.1 mmol of TMOS, 36.4 mmol of H2O (water-to-monomer ratio, rw/m ) 4), and the corresponding R19 mols (Table 1) dissolved in methanol (33 mmol). After 60 min of stirring, the mixture was transferred to a polystyrene cuvette and covered with aluminum foil. After gelation, the aluminum foil was perforated to allow the slow evaporation of the solvents until the formation of dried xerogels. The R19 molar concentration of the resulting gels (gels 1-7) is given in Table 1. Fluorescence Spectroscopy and Lifetimes. Multifrequency modulation, phase analysis, and fluorimetric measurements were performed at 25 °C on a 48000s (T-Optics) spectrofluorometer from SLM-Aminco.20 Emission and excitation spectra were recorded by reflection (front face mode) in the backward direction21 on pills of 50 µm thickness prepared from finely ground samples, as described elsewhere.22 Steady-state fluorescence anisotropy was measured on a bulk sample (gel 1) with cubic shape and 0.5 cm path length, as described elsewhere.23 Lifetimes were measured by reflection (front face mode) in the backward direction on finely ground samples packed into quartz cells with 1 mm path length, as described elsewhere.24 The goodness of the model used for the analysis of the phase and modulation data, and hence the accuracy of the lifetime values measured, was determined according to the lower χR2 value found in each individual measurement.25,26 The lower χR2 values were always obtained by fitting of the data to a single component rather than to two (or three) components. An uncertainty of 0.5 in the resolution of the phase and 0.005 in the resolution of the modulation was applied for the calculation of χR2. UV-Vis Spectroscopy. A Varian UV-vis spectrophotometer model 2300 was used to measure the absorption spectra (reflectance mode) of samples finely ground and packed in quartz cells with 1 mm path length. The absorption spectrum in transmittance mode was measured on gel 1 (a bulk sample with 0.5 cm path length) to obtain the extinction coefficient ( ∫(ν) dν) of R19 in the porous cage of a sol-gel silica glass. (20) (a) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Plenum Press: New York, 1983; Chapter 2, p 20. (b) Steiner, R. F. Topics in Fluorescence Spectroscopy, Volume 2: Principles; Lakowicz, J. R., Ed.; Plenum Press: New York, 1991; Chapter 1. (c) Lakowicz, J. R.; Laczko, G.; Cherek, H.; Gratton, E.; Limkeman, M. Biophys. J. 1984, 46, 462. (21) Oelkrug, D. Topics in Fluorescence Spectroscopy, Volume 4: Probe Design and Chemical Sensing; Lakowicz, J. R., Ed.; Plenum Press: New York, 1994; Chapter 8. (22) Ferrer, M. L.; del Monte, F.; Levy, D. Langmuir 2001, 17, 4812. (23) del Monte, F.; Ferrer, M. L.; Levy, D. J. Mater. Chem. 2001, 11, 1745. (24) del Monte, F.; Mackenzie, J. D.; Levy, D. Langmuir 2000, 16, 7377. (25) Gratton, E.; Linkeman, M. Biophys. J. 1983, 44, 315. (26) Bevington, P. R. Data reduction and Error Analysis for the Physical Sciences; McGraw-Hill: New York, 1969; p 336.

Figure 1. Excitation (), λem ) 600 nm) and emission ((, λex ) 480 nm) spectra of R19-doped gel 1. Inset: Molecular structure of R19.

Results and Discussion The spectroscopic properties of R19-doped gel 1 can be ascribed to nonaggregated species; that is, no signs of aggregation are observed in the excitation spectra shown in Figure 1. The analysis of the frequency-dependent phase and modulation data allows the determination of a single fluorescent lifetime for gel 1, which indicates the homogeneous dispersion of the dye within the porosity of the silica glass. The value of the lifetime in gel 1 was similar to that reported previously for R6G in sol-gel silica glasses (∼3.85 ns7). Rhodamine lifetime values within the 3-4 ns range are characteristic of adsorbed molecules assuming maximum interaction between the dye and the substrate.27 In the adsorbed state, such a maximum interaction limits the capability of the amine groups to rotate with the subsequent reduction of the deactivation processes or quenching (S1 f S0 internal conversion processes).28,29 The high 〈r〉 value recovered from the steady-state anisotropy measurement of gel 1 (0.336) is indicative of a slowed or partially hindered dynamic,23 which also denotes the dye allocation in a restricted environment, that is, adsorbed on the porous surface. Nevertheless, the measured 〈r〉 value was below the fundamental anisotropy value expected for a vitrified system, which is indicating that the dye is not totally constrained by the pore cage, and it still keeps a certain degree of mobility.30 A detailed description of the adsorption process of different xanthene-like structures on the porous surface of sol-gel silica glasses has recently been published.31 The situation described above (e.g., R19 molecules adsorbed on the porous surface but not totally constrained by the pore cage) must allow the formation of either fluorescent J-dimers or nonfluorescent H-dimers at increased R19 concentrations upon the rearrangement of the molecules. Silica glasses with R19 dye concentrations ranging from 4.0 × 10-6 M (gel 1) to 6.4 × 10-4 M (gel 8) were studied to elucidate such an aggregation process. To date, the best description of the aggregation behavior of dye chromophores is based on the exciton-splitting theory.17-19 The relative intensity of the absorption bands can provide structural information about the dimer regarding its simplest geometry model (e.g., the chromophore spacing and the relative spatial arrangement of (27) Snyder, L. R. J. Phys. Chem. 1963, 67, 2622. (28) Itho, K.; Honda, K. Chem. Phys. Lett. 1982, 87, 213. (29) Vogel, M.; Rettig, W.; Sens, R.; Drexhage, K. H. Chem. Phys. Lett. 1988, 147, 461. (30) Dale, R. E. Time-Resolved Fluorescence Spectroscopy in Biochemistry and Biology; Cundall, R. B., Dale, R. E., Eds.; NATO ASI Series A: Life Sciences, Vol. 69; Plenum Press: New York, 1983; pp 605-606. (31) Ferrer, M. L.; del Monte, F. Langmuir 2003, 19, 650.

2784

Langmuir, Vol. 19, No. 7, 2003

Ferrer et al. Table 2. Maximum of Excitation (λmex) and Emission (λmem) Spectra, Lifetimes (τ),a,b Experimental Reduced Chi-Squared (χR2) for R19 Gels 1-8, and Experimental Adsorption Angle (O) for R19 Gels 2-6c gel

λmex (nm)

λmem (nm)

τ (ns)

χR2

φ (deg)

1 2 3 4 5 6 7 8

527 527 528 528-508 529-508 528-508 528 528

544 546 547 552 556 558 560 561

3.8 ( 0.2 4.3 ( 0.2 4.5 ( 0.2 5.0 ( 0.2 5.4 ( 0.2 4.9 ( 0.2 4.6 ( 0.2 4.4 ( 0.2

1.1 1.0 1.0 0.9 1.2 1.1 1.0 1.1

monomer 48.0 ( 0.8 50.0 ( 0.8 51.9 ( 0.8 53.6 ( 0.8 H dimers H dimers H dimers

a The emission wavelength used to record the excitation spectra was 600 nm. b The excitation wavelength used to record the emission spectra and to measure the lifetime values was 480 nm. c Errors were calculated as described elsewhere (refs 25 and 26).

Figure 2. Exciton band energy diagram for molecular dimers in (a) a parallel plane twist angle configuration (φ > 54.7°) and in (b) an oblique in-plane configuration (φ < 54.7°), from Kasha et al. (ref 17). Dipole-forbidden transitions are indicated by dashed lines, and dipole-allowed transitions by solid lines. Reprinted with permission; copyright 1965 by Blackwell. Figure 4. Absorption spectra (in reflectance mode) of R19doped gels 4-8.

Figure 3. Normalized excitation spectra (λem ) 600 nm) of R19-doped gel 1 ()), gel 3 (O), gel 5 (4), and gel 8 (dashed line).

the transition dipole moments of the molecules forming the dimers, Figure 2). The band placed at higher energies is defined as the H-band, while that at lower energies is defined as the J-band. The in-plane oblique angle configuration exists for φ < 54.7°, and its dipole arrangement allows the S0 f S1 transition to both the J-band and the H-band. For φ > 54.7°, there is a rearrangement of the molecules to a parallel plane twist angle configuration, whose dipole arrangement only allows the S0 f S1 transition to the H-band.17-19 Figure 3 shows the excitation spectra experimentally found for R19 gels 1-5. The increase of concentration gave rise to both the red shift (up to 529 nm) and the appearance of a shoulder at the blue edge (∼508 nm) of the main excitation band at 527 nm (Table 2). As mentioned above, the splitting of the main excitation band of nonaggregate species (gel 1) indicates the formation of dimers in oblique configuration (Figure 2). Note that the observance of the splitting in an excitation spectrum denotes the fluorescent character of the dimers formed in gels 2-5. Further increase of the R19 concentration (gels 6-8) resulted in the progressive intensity decrease of the split bands (i.e., blue shift of the band at 529 nm and

Figure 5. Emission spectra (λex ) 480 nm) of R19-doped gel 1 ((), gel 3 (b), gel 4 (+), gel 5 (2), and gel 8 (9). The fluorescence intensity is normalized. Inset: Fluorescence intensity of gels 1-8 versus R19 concentration.

intensity decrease of the band at 508 nm), which indicates the vanishing of the corresponding fluorescent species. The absorption spectra of R19 gels 6-8 (Figure 4) showed the intensity increase of the shoulder placed at higher energies in the excitation spectra of gels 2-5 (∼508 nm) to become the main band of the absorption spectrum of gel 8. The behavior shown by the excitation and the absorption spectra of gels 6-8 clearly reveals the conversion from fluorescent to nonfluorescent dimers accompanied by a configuration change (from in-plane oblique angle to parallel plane twist angle) through the rearrangement of the monomeric constituents. The fluorescent character of the dimers formed in gels 2-5 was also observed by a red shift of the emission spectra from gel 1 to gel 5 (Figure 5). The fluorescent character of dimers is mostly due to the allowed transition from the J-dimer level to the ground level, whose transition energy is lower than that of nonaggregates.13-15 Correlation of

Fluorescent Dimers from Dye Aggregation

Langmuir, Vol. 19, No. 7, 2003 2785

longitudinal axis of the molecules and the adsorption surface (φ, in Figure 2) with the radiative rate constant of the fluorescent dimers (krD) and of the monomer unit (krM):13,14

krD ) 2 krM cos2 φ

Figure 6. Extinction coefficient of R19-doped gel 1 (∫(ν) dν ) 2.22 × 108 M-1 cm-1).

the fluorescence emission intensity measured as the total area under the emission spectra with dye concentration was also found up to gel 5,9,10 while over that concentration the fluorescence emission decreases as a consequence of the appearance of aggregates that contribute to the quenching of the excited states through intersystem crossing processes (Figure 6).32-34 The appearance of nonfluorescent H-dimers in gels 6-8 is in good concordance with results from absorption and excitation spectra reported above. Both the red shift of the emission spectra and the linear increase of the fluorescence intensity with the surface density of adsorbed chromophores up to gel 5 have been previously reported as consequences of the increase of the polarizability of the dye surrounding environment that results from the close packaging of the xanthene moieties in dimers and aggregates.9,10,13 The linear increase of the fluorescent intensity observed in gels 1-5 also rules out the occurrence of any reabsorption process.35-37 Otherwise, fluorescence intensity would decrease with the greater overlapping between absorption and emission spectra.36 Furthermore, neither the excitation nor the absorption spectra of gels 2-7 show any trace of reabsorption, most likely due to the experimental conditions used in this paper for the recording of the emission spectra (e.g., reflection mode (front face) on pills of 50 µm thickness) which make negligible the occurrence of light reabsorption.38 This issue is worth pointing out since part of the photophysic behavior described above could be explained in terms of light reabsorption processes; for example, red shift of the emission spectra due to the overlapping of the absorption and the emission spectra, and increase of the lifetime values by radiating trapping phenomena.35-37 Knowledge regarding the factor that governs the conversion from fluorescent J-dimers to nonfluorescent H-dimers at the R19 concentration of gel 5 is required at this stage of the work. The measurement of the lifetime of R19 in gels 1-5 and the application of the exciton theory17-19 to the resulting values should help to clarify this point. Regarding lifetime values, a gradual increase was observed from gel 1 to gel 5, in concordance with the increase of the fluorescence intensity described above. Equation 1 correlates the angle formed between the (32) Ballet, P.; Van der Auweraer, M.; De Schyver, F. C.; Lemmtyinen, H.; Vuorimaa, E. J. Phys. Chem. 1996, 100, 13701. (33) Liang, Y.; Moy, P. F.; Poole, J. A.; Ponte Goncalves, A. M. J. Phys. Chem. 1984, 88, 2451. (34) Nasr, C.; Liu, D.; Hotchandani, S.; Kramat, P. V. J. Phys. Chem. 1996, 100, 11054. (35) Hammond, P. R. J. Chem. Phys. 1979, 70, 3884. (36) Lo´pez-Arbeloa, I. J. Photochem. 1982, 18, 161. (37) El-Daly, S. A.; Okamoto, M.; Hirayama, S. J. Photochem. Photobiol., A 1995, 91, 105. (38) Kawahigashi, M.; Hirayama, S. J. Lumin. 1989, 43, 207.

(1)

As described above, the main mechanism of nonradiative deactivation (e.g., rotation about the xanthene-amine bond) is minimized in the situation in which R19 molecules are allocated in gels 1-5 (e.g., adsorbed on the porous surface). Under these circumstances, the radiative rate constant of both the monomer and the fluorescent dimers can directly be approximated to the inverse of their lifetime values (R19 lifetime of gel 1 and gels 2-5, respectively).39 The calculated adsorption angles for gels 2-5 are reported in Table 2. The φ angle calculated for gel 5 is at the limiting value accepted for the formation of J-dimers (φ ∼ 53°, Table 2). A further increase of concentration (gels 6-8) gave rise to a decrease of the lifetime values, which is indeed reflecting the appearance of nonfluorescent Hdimers capable to quench the excited states through intersystem crossing processes.32-34 Any trace of fluorescence should vanish by a complete J- to H-dimer conversion. The residual fluorescence intensity observed for gels 6-8 (Figure 6) and their corresponding lifetime values (Table 2) are indicating the presence of a distribution of dimer geometries. Actually, large red shifts of the emission spectra (up to 20 nm) have been reported as indicative of the superposition of a large variety of fluorescent, weakly fluorescent, and nonfluorescent species.13 A narrow distribution of dimer geometries (less than 7 nm) has only been reported in conditions with a high degree of orientational order.40 The experimental red shifts found in gels 2-5 (12 nm) are between those values, which indicates a moderate distribution of dimer geometries. Unfortunately, the discrimination of two or more different lifetimes within such a distribution, each one corresponding to a dimer geometry, as well as their partial contributions to the fluorescence intensity, can hardly be attempted with our experimental setup. Therefore, the measured lifetimes reported above, and hence the adsorption angles calculated from eq 1, must be taken as averaged values for such a distribution of dimer geometries, but never as discrete values representing a single dimer geometry. The remaining parameters (R and R, Figure 2) required to define the geometry of the fluorescent J-dimers can be obtained through the application of the exciton theory.17-19 Exciton theory has been typically applied on the absorption spectra of the aggregates.41-43 The fluorescent character of the R19 dimers described above must allow the application of the exciton theory to the excitation spectra of gel 5. Thus, the R angle between the monomer units in the dimer was calculated from the ratio between the areas of the long-wavelength (A1) and the short-wavelength (A2) dimer excitation bands (eq 2). The interaction energy (U, in cm-1) between the monomers forming the aggregate was calculated from the long-wavenumber (ν2, in cm-1) and the short-wavenumber (ν1, in cm-1) dimer excitation bands (eq 3). The ratio between the areas of the long(39) Karstane, T.; Kobe, K. J. Phys. Chem. 1980, 84, 1871. (40) Kikteva, T.; Star, D.; Zhao, Z.; Baisley, T. L.; Leach, G. W. J. Phys. Chem. B 1999, 103, 1124. (41) Lo´pez-Arbeloa, F.; Martı´nez-Martı´nez, V.; Ban˜uelos-Prieto, J.; Lo´pez-Arbeloa, I. Langmuir 2002, 18, 2658 and references therein. (42) Lo´pez-Arbeloa, F.; Tapia-Este´bez, M. J.; Lo´pez-Arbeloa, T.; Lo´pez-Arbeloa, I. Langmuir 1995, 11, 3211. (43) Bojarski, P.; Matczuk, A.; Bojarski, C.; Kawski, A.; Kuklinski, B.; Zurkowska, G.; Diehl, H. Chem. Phys. 1996, 210, 485.

2786

Langmuir, Vol. 19, No. 7, 2003

Ferrer et al.

wavelength and the short-wavelength dimer excitation bands as well as the wavelength values were experimentally calculated as described elsewhere.43

tan2 (R/2) ) A1/A2

(2)

U ) (ν2 - ν1)/2

(3)

For the oblique configuration, the separation distance between the molecules (R, in Å) can be calculated from eq 4, where νm (cm-1) and ∫(ν) dν (M-1 cm-1) are the wavenumber at the maximum intensity and the area of the R19 gel 1 absorption band, respectively (Figure 6).

R)

(

1.85 × 102

)

∫(ν) dν [cos(R) + 3 sin(R/2)] 1/3 Uνm

(4)

The calculated angle between the monomer units (R) for R19 gel 5 was 64.4°. The goodness of the calculated data is corroborated by the sum R + 2φ ) 180° (Figure 2). The R value obtained for R19 gel 5 was 8.92 Å, in the range of that reported for R6G and between those for RB and R110, at similar dye concentrations.24 The observed tendency must be ascribed to the increase of the steric hindrance between the monomer constituents of aggregates in an oblique configuration as the sizes of the groups linked to the nitrogen increase (-N(CH2CH3)2 for RB, -NH(CH2CH3) for R6G and R19, and -NH2 for R110). The R value reported for R3B in an oblique configuration keeps such a tendency, even though it was obtained through the application of the exciton theory to the absorption spectra.42 Such a concordance corroborates the validity of the approach followed in this work, that is, the

application of the exciton theory to the excitation spectrum. Moreover, the larger R value found for R19 than that reported for rhodamines in a parallel plane twist angle configuration41 corroborates the in-plane oblique angle as the preferred configuration of the R19 fluorescent dimers studied in this work. Conclusions Absorption, excitation, and emission spectroscopy as well as lifetime measurements have demonstrated the capability of fluorescent tools to provide information regarding the aggregation process of R19 in sol-gel silica glasses as a consequence of the chemical interactions that the molecules establish among themselves and with their surrounding environment, for example, the adsorption porous surface. The application of the exciton theory to the fluorescent J-dimers has allowed the determination of the spatial arrangement of the molecules (e.g., in-plane oblique angle configuration) and the structural parameters that define such an arrangement (R, φ, and R). Fluorescence has also allowed observation of how the progressive adsorption of molecules on the porous surface causes the increase of the adsorption angle (φ) that gives rise to a rearrangement of the adsorbed species with the subsequent change of the fluorescent properties, for example, from fluorescent J-dimers in an in-plane oblique angle configuration to nonfluorescent H-dimers in a parallel plane twist angle configuration. Acknowledgment. The authors are grateful to CICYT for Research Grant MAT 2001-5073-E. Maria L. Ferrer is grateful to CSIC for a I3P research contract. Dr. David Levy is acknowledged for valuable support. LA026685T